A continuous casting apparatus can comprise a water cooled mold which is provided with a tundish having an adjustable outlet for a melt and an extraction, strand advancing or withdrawal mechanism for the solidified product or strand.
The mold is provided with a measuring mechanism for measurement of the state of filling of the mold which controls the speed of the withdrawal mechanism, or its withdrawal rollers if it is so equipped during free running operation, or the opening of the tundish outlet.
Before the start of casting operation the lower mold opening is closed with a starting "head" formed by a dummy bar; when the molten metal is then supplied the molten steel fuses with the head onto which cold scrap can be applied to facilitate the fusion.
By the water cooled mold so much heat is drawn from the hot flowing metal that a rigid shell of the strand forms against the mold wall. As soon as the state of filling of the mold is such that the level of the metal has reached a predetermined height, the withdrawal mechanism is switched on and the cooled strand is drawn off while continuously solidifying. The continuous casting can then be subjected to classical continuous metal casting control process.
An automatic casting process has problems since the measuring mechanism is mounted in the vicinity of the desired (setpoint) height of the bath level and useable measurements are only transmitted when the bath level approaches its desired level.
During casting vigorous bath motion and splashing occurs, which can result in erroneous measurements of the state of filling or make difficult the interpretation of such measurements.
An automatic startup of the casting process is described in German Patent Document No. 32 21 708. The process described there for filling the mold is characterized by an intermittent automatic opening of the outlet for dollopwise filling of the mold over a time interval from the beginning of melt feed until the filling of the space in the vicinity of the starting head and/or until the decay of the initial strong bath motions in the mold.
From the Belgian Patent Document BE No. 704,306 it is known to fill the mold continuously in a first stage of a casting process and, when the height of the reservoir metal level is detected by the measuring mechanism to again close the outlet. Subsequently the withdrawal mechanism is started and the outlet is placed under control of a P-governing branch of an electronic control circuit, i.e. a proportional (P) controller.
However one does not choose this kind of process when casting articles with a small cross sectional area. Here the volume filled by the steel is very small; the thermal inertia of the steel-mold system is drastically reduced. Also the stopper which closes the outlet is brought from the open to the closed positions in about 0.8 seconds by the customary adjusting drive. Thus during the measurement of a first value by the measuring mechanism, the outlet can only close half way in the available time.
In dollopwise filling with very cool charges (say, about 20.degree. C. above the melting point) there is the great danger that with small stopper displacement the heat input or delivery to the outlet and mold per stopper stroke will become too small and the outlet or the downcomer or immersion tube will become clogged.
In casting after changing the immersible tube, this danger is even greater since the immersible tube is not preheated. It can also be observed with very inferior grades of steel during a pause in a stroke of the stopper that the steel will congeal in the immersible tube and in the next stroke the immersible tube will be broken loose from the reservoir. To treat this condition the stopper stroke can be lengthened at the outlet so that the largest possible quantity of heat per stroke is delivered to the immersible tube.
This approach is, however, limited since a lengthening of the open time automatically means an increase of the stopper stroke and a large opening at the outlet at the time and in the region of the level measurement which can lead to a steel overflow. Also in this process the indefinite stopper position on reaching the switchover point is not beneficial for adjustment of the apparatus from the initial pouring stage to the regulated stage.
With charges having very high Al-content(without CaSi) these problems are still greater. Here the alumina builds up at the outlet on every closing of the stopper so that the stopper null position is displaced upwardly. Because of a nonuniform structure of the alumina deposit the stopper can no longer be tightly closed. An even higher energy input must be used here in order to avoid clogging the apparatus.